Lead-acid batteries are excellent in high-rate discharge performance, and are still widely used industrially, considering long-term reliability and costs. Such lead-acid batteries are used particularly as a battery for starters for automobiles, and as various industrial and business backup power sources. Future trends in lead-acid batteries of the two fields, i.e., starter batteries and backup power sources, can be considered as in the following.
In the field of backup power sources, there is a growing tendency to replace lead-acid batteries with nickel-metal hydride storage batteries or non-aqueous electrolyte secondary batteries (lithium ion battery). Main reasons for the replacement are demands for downsizing power sources by replacing lead-acid batteries with batteries having further higher energy density, and demands for environmentally friendly batteries instead of batteries using lead in view of environmental load. Regarding the lead-acid battery for automobile starters, although there is no significant step for the replacement, examination for practical use of lithium ion batteries for power sources for stop-idling vehicles has been started. In hybrid cars such as PRIUS (product name), nickel-metal hydride storage batteries are already used.
Particularly, lithium ion batteries have been used actually as a small mobile power source for 10 or more years, and during the period, techniques for securing higher safety and reliability have been achieved without sacrificing its characteristic high energy density. Lithium ion batteries have also been achieving cost reduction for industrial application. From these points mentioned above, in the above two fields, the trend towards the use of lithium ion batteries to replace lead-acid batteries is considered to further grow in the future.
Lithium ion batteries used as main power sources of mobile electronic devices have the following components. For the positive electrode active material to be included in the positive electrode, lithium cobaltate (LiCoO2), lithium nickel manganese cobalt oxide (LiNi1/2-xMn1/2-xCOxO2) or spinel lithium manganese oxide (LiMn2O4) is used. These positive electrode active materials have a voltage of 4 V or more relative to lithium. For the negative electrode, carbon materials are generally used, which achieve a lithium ion battery of 4 V class when being used with the above positive electrode. Other than such types, various types of batteries have been proposed. For example, in view of higher safety, use of for example olivine lithium iron phosphate (LiFePO4) in the positive electrode is proposed, and in view of further higher energy density, use of a silicon alloy in the negative electrode is proposed.
On the other hand, in battery packs in which a plurality of batteries are connected in series or in parallel, and in so-called battery modules in which a plurality of unit cells are installed in a single container, there is a definitive difference between aqueous solution-type batteries such as lead-acid batteries and nickel-metal hydride batteries, and non-aqueous electrolyte-type batteries such as lithium ion batteries. Battery modules are the batteries such as lead-acid batteries for 12 V vehicles, and prismatic nickel-metal hydride batteries for hybrid cars. The above battery modules are obtained, for example, by forming a plurality of spaces in a resin-made container with partitions, installing a unit cell in each space, and connecting the unit cells in series via holes passing through the partitions.
In the above aqueous solution-type batteries, over-charge protection is achieved by so-called Neumann mechanism using the electrolysis of water. For example, nickel-metal hydride storage batteries are designed so that the negative electrode capacity is larger than the positive electrode capacity. With such a design, the irreversible capacity necessary for the formation of the positive electrode is charged in the negative electrode at the initial charging, and as a result, the negative electrode becomes able to take charge of both the discharge reserve and charge reserve. By forming the battery with such a design, oxygen generated from the positive electrode at the time of overcharged state can be adsorbed by the negative electrode, and hydrogen generated at the time of reverse charging (over-discharge) can be adsorbed by the negative electrode.
Therefore, in battery modules and battery packs, there is no need for controlling the charging in each unit cell having a voltage of 1.2 V, and a group of a plurality of unit cells in series having a voltage such as 6 V, 12 V, or 24 V can be controlled as a single unit for the charging. However, in lithium ion batteries currently in practical use, the charging control has to be carried out in each unit cell for the overcharge protection, since the principle of the Neumann mechanism mentioned in the above does not work. Thus, when lithium ion batteries are used in the above two fields including a plurality of unit cells, there is a problem of cost increase since the charging has to be controlled for each unit cell. Although it may be possible to observe a battery voltage in each unit cell and to control the current only at the both ends of the above group, this is not so effective since the charging has to be ended at the unit cell with the least capacity.
In view of the above conventional technology, the present invention mainly focuses on achieving a non-aqueous electrolyte secondary battery which can achieve particularly the overcharge protection by internal chemistry without depending on external electronic circuit, as a replacement for lead-acid batteries.
Relating to such non-aqueous electrolyte secondary batteries, for example, Japanese Laid-Open Patent Publication No. Hei 8-22841 has proposed a non-aqueous electrolyte battery system which can endure over-discharge in a battery pack (integrated battery). In this publication, the spinel-type lithium-containing metal oxide is used for the active material of both of the positive electrode and the negative electrode: to be specific, lithium manganese oxide (Li1.05Mn0.95O4) is used for the positive electrode active material, and lithium titanium oxide (Li1.035Ti1.965O4) is used for the negative electrode active material. Since Ti has an average valence of 3.5, Li1.035Ti1.965O4 in the negative electrode can be oxidized by releasing Li at the time of over-discharge (reverse charging) until Ti has a valence of four. At the same time, Japanese Laid-Open Patent Publication No. Hei 8-22841 describes that since Mn has an average valence of 3.6, Li1.05Mn1.95O4 in the positive electrode can adsorb Li until Mn has a valence of three. Therefore, reversibility of the active material is not deteriorated even in the over-discharge (reverse charging) state.
However, Japanese Laid-Open Patent Publication No. Hei 8-22841 does not mention the overcharge protection. Regarding the balance between the positive electrode active material capacity and the negative electrode active material capacity as well, no particular consideration seems to be taken, even in view of the description in Examples. Also, there is no particular matter to be noted regarding non-aqueous electrolyte, and “a mixed solution of ethylene carbonate (EC) and diethylcarbonate (DEC) in which LiPF6 was dissolved in 1 mol/liter” shown in Example is currently in general use in lithium ion batteries.
Additionally, in Journal of Electrochemical Society, 152, 2390 (2005) by J. R. Dahn et al., techniques for overcharge protection in lithium ion batteries are proposed. This relates to the technique called redox shuttle, and is characterized in that an additive with redox capability is added to a generally used non-aqueous electrolyte. The journal notes that the additive is oxidized in the positive electrode at the time of overcharged state, the oxidized additive diffuses into the negative electrode, and reverse reactions occur in the negative electrode to consume overcharge current. To be specific, the journal shows that in the system using the positive electrode active material including LiFePO4 and the negative electrode active material including Li4/3Ti5/3O4, 2,5-di-t-butyl-1,4-dimethoxybenzene is promising for the redox shuttle (additive).
However, although this concept of the redox shuttle has been proposed so far, it has a critical problem: the reaction speed of the shuttle is slow, and in batteries in practical use, the overcharge rate that can be used for the overcharge protection cannot be secured. Also, with the electrochemical reaction of oxidation and reduction, heat reaction that cannot be disregarded occurs, which renders the redox shuttle concept unrealistic.
Japanese Laid-Open Patent Publication No. Hei 7-335261 has disclosed a battery using the positive electrode including lithium cobaltate (LiCoO2), and the negative electrode including lithium titanate (Li4/3Ti5/3O4). Japanese Laid-Open Patent Publication No. Hei 7-335261 mentions the balance between the positive electrode capacity and the negative electrode capacity, and according to this publication, the negative electrode capacity is preferably set to the rate of 0.6 to below 1.0 to the positive electrode capacity, and the battery capacity is preferably regulated by the negative electrode capacity (negative electrode capacity regulation). However, the publication just mentions that the negative electrode capacity regulation is preferable in view of cycle life, and there is no mention regarding protection for overcharge and overdischarge.
Further, Japanese Laid-Open Patent Publication No. Hei 10-27609 shows a battery using the following: the negative electrode active material including lithium or a lithium alloy, or a spinel structure lithium-titanium oxide; the positive electrode active material including a spinel structure lithium-manganese oxide, Li4/3Mn5/3O4; and a non-aqueous electrolyte in which LiN(CF3SO2)2 is dissolved in a solvent mixture of two or more components including ethylene carbonate. However, Japanese Laid-Open Patent Publication No. Hei 10-27609 merely intends to improve cycle life and storage characteristics, by optimizing the electrolyte.
Also, for the prior art relating to the replacement for lead-acid batteries as mentioned above, for example, Japanese Laid-Open Patent Publication No. 2003-323893 and Japanese Laid-Open Patent Publication No. 2005-142047 may be mentioned. And in Japanese Laid-Open Patent Publication No. Hei 10-106626, aiming to achieve a non-aqueous electrolyte with a high conductivity, an electrolyte including acetonitrile as a solvent is proposed.
As mentioned above, in a battery pack including a plurality of unit cells connected in series or in parallel, and in a battery module in which a plurality of unit cells are installed in single container, there is a definitive difference between aqueous solution-type batteries in which Neumann mechanism works (such as lead-acid batteries and nickel-metal hydride batteries,) and non-aqueous electrolyte-type batteries in which Neumann mechanism does not work (such as currently used lithium ion batteries). For example, in lead-acid batteries for automobiles, charging and discharging can be controlled by controlling only the voltage of at both ends of a 12 V battery, in which six unit cells connected in series are included in a container. However, in lithium ion batteries including a plurality of unit cells, when there is a difference between the capacities of the unit cells, even a minute difference, charging and discharging cannot be controlled by controlling only the voltages at both ends of the plurality of unit cells, and overcharge and overdischarge occur unless the unit cell is controlled, leading to a significant decline in safety and reliability. When lithium ion batteries are used for industrial backup power sources which require high voltage and output, and for power sources for automobiles, it is highly costly to control each unit cell.
The present invention is for solving these problems, and aims to provide a non-aqueous electrolyte secondary battery that can achieve particularly the overcharge protection with its internal chemistry without depending on an external electronic circuit, for replacing lead-acid batteries. To be more specific, the present invention aims to provide a non-aqueous electrolyte secondary battery with a battery chemistry that enables the following: protection for both overcharge and overdischarge (reverse charging), by appropriately selecting the combination of the positive electrode active material, the negative electrode active material, and the electrolyte, and the balance between the positive electrode capacity and the negative electrode capacity; and simplification of the control over battery packs and battery modules.